For a nucleic acid (e.g., a gene) transfection into cells, there have already been known methods which make use of cationic lipid alone, or a complex formed between the nucleic acid and liposomes containing cationic lipid (see, for example, Patent Document 1). In such a method, commercially available reagents such as “Lipofectamine,” “Lipofectin,” “Transfectam,” “Genetransfer,” and “Lipofectamine 2000” are employed.
However, these commercially available reagents pose problems as described below. (a) Such a commercially available reagent exhibits poor storage stability, or fails to attain intracellular transfection and expression of a gene by use of, for example, liposomes at high reproducibility. (b) Such a commercially available reagent is very unstable in serum (fetal bovine serum) added to a medium for cell culture, and thus the reagent requires an intricate process for a nucleic acid transfection (i.e., a serum-containing medium in which cells are cultured is temporarily replaced with a serum-free medium, and then the serum-free medium is replaced with the serum-containing medium after a nucleic acid transfection). Recently, it has been become clear that such a commercially available reagent is very unstable also in blood or a living body. (c) Most of these commercially available reagents (e.g., Lipofectamine, Lipofectin, and Lipofectamine 2000) are provided only in the form of a dispersion containing lipid dispersed in water. Therefore, an aqueous solution of a gene is added to such a reagent for the gene transfection. However, in such a case, liposomes encapsulating the gene fail to be produced, although a complex in which the gene is bound to the outside of liposomes can be produced. Lipofectamine 2000 should not be excessively stirred or shaken and must be handled with the greatest care for preventing formation of cationic lipid peroxide. (d) Such a commercially available reagent exhibits very potent cytotoxicity.
Thus, many problems are involved in some commercially available reagents for a nucleic acid (e.g., a gene) transfection into cells by use of cationic lipid singly or cationic-lipid-containing liposomes.    Patent Document 1: JP-A-1990-135092